Methanol carbonylation system having absorber with multiple solvent options

ABSTRACT

A methanol carbonylation system  10  includes an absorber tower  75  adapted for receiving a vent gas stream and removing methyl iodide therefrom with a scrubber solvent, the absorber tower being coupled to first and second scrubber solvent sources  16, 56  which are capable of supplying different first and second scrubber solvents. A switching system including valves  90, 92, 94, 96, 98  alternatively provides first or second scrubber solvents to the absorber tower and returns the used solvent and sorbed material to the carbonylation system to accommodate different operating modes.

CLAIM FOR PRIORITY

This non-provisional application claims the benefit of the filing dateof U.S. Provisional Patent Application Ser. No. 61/125,791, of the sametitle, filed Apr. 29, 2009. The priority of U.S. Provisional PatentApplication Ser. No. 61/125,791 is hereby claimed and the disclosurethereof is incorporated into this application by reference.

TECHNICAL FIELD

The present invention relates to acetic acid production and, inparticular, to a methanol carbonylation system with a light endsabsorber adapted to use different scrubber solvents and return the usedsolvent to the carbonylation system.

BACKGROUND OF THE INVENTION

Acetic acid production by way of methanol carbonylation is known in theart. Generally speaking, a methanol carbonylation production lineincludes a reaction section, a purification section, light ends recoveryand a catalyst reservoir system. In the reaction section, methanol andcarbon monoxide are contacted with rhodium or iridium catalyst in ahomogenous stirred liquid phase reaction medium in a reactor to produceacetic acid. Methanol is pumped to the reactor from a methanol surgetank. The process is highly efficient, having a conversion of methanolto acetic acid of typically greater than 99 percent. The reactionsection also includes a flash vessel coupled to the reactor whichflashes a draw stream in order to remove crude product from the reactionsection. The crude product is fed to a purification section whichincludes generally a light ends or stripper column, a drying column,auxiliary purification and optionally a finishing column. In theprocess, various vent streams containing light ends, notably methyliodide, carbon monoxide and methyl acetate are generated and fed to thelight ends recovery section. These vent streams are scrubbed with asolvent to remove the light ends which are returned to the system ordiscarded.

In a traditional, Monsanto methanol carbonylation plant, a high pressureand low pressure absorber are included wherein acetic acid is used asthe scrubber solvent. The acetic acid solvent must subsequently bestripped of light ends, usually in another purification column so thatthe acid is not wasted. Such columns are expensive because they must bemade of a highly corrosion resistant material such as zirconium alloysand so forth. Moreover, stripping light ends from the acid requiressteam and contributes to operating expense.

Nevertheless, using acetic acid as a scrubber solvent is widespread inthe carbonylation art generally. See for example, U.S. Pat. No.5,502,243 to Waller et al., entitled “Hydrocarbonylation of DimethylEther”. Note the disclosure at FIG. 3, and the discussion at Cols. 8 and9 concerning operation of an absorber 321. A cool acetic stream 323passes downwardly through this absorber and absorbs any residualco-products and volatile catalyst components from the vent gas.

So also, there is disclosed in U.S. Pat. No. 4,241,219 to Wan, entitled“Treatment of Carbonylation Effluent”, a method of recovering volatilecomponents by contact with a scrubbing solvent recovered from thereaction mixture in the same production line. See Col. 2, lines 15-30wherein it is noted that acetic anhydride, ethylidene diacetate, aceticacid, or mixtures of them can be used as a vent gas scrubber solvent.

Methanol has been suggested for use as a scrubber solvent in connectionwith a methanol carbonylation process. In this regard, see U.S. Pat. No.5,416,237 to Aubigne et al., entitled “Process for the Production ofAcetic Acid”. It is noted in the '237 patent that noncondensables from aflash tank vapor overhead may be scrubbed countercurrently with chilledmethanol. The methanol scrubber solvent residual stream is added to puremethanol and then used as feed to the reactor. See Col. 9, lines 30-42.If the reactor is not consuming the residual stream, it must be storedseparately or purified again contributing to capital expense andoperating costs.

Chinese Patent Application Publication No. 200410016120.7 discloses amethod for recovering light components in vent gas from production ofacetic acid/acetic anhydride by way of scrubbing with methanol andacetic acid. The system disclosed in the Publication No. 200410016120.7discloses a two stage absorption arrangement wherein vent gas is treatedsequentially with two different absorbents in a two stage system. Noteparticularly FIG. 2. Another system is seen in an industrial publicationentitled “Process of 200 ktpa Methanol Low Press Oxo Synthesis AA”(SWRDICI 2006) (China). In this research publication there is shown avent gas treatment system including a high pressure absorber as well asa low pressure absorber. Both absorbers of this system are operatedutilizing methanol as a scrub fluid.

While there have been advances in the art, known methods of scrubbingvent gases in methanol carbonylation systems typically involve multipleabsorber towers which are expensive to fabricate and operate. Inaccordance with the invention, there is provided an improved methanolcarbonylation system with an absorber capable of using differentsolvents. The inventive system reduces both capital requirements andoperating costs as compared with conventional systems.

SUMMARY OF THE INVENTION

There is provided a carbonylation system for making acetic acid havingan absorber tower with multiple scrubber solvent options for treatingvent gas. The absorber recovers methyl iodide and other volatiles suchas methyl acetate vapor from the vent gas with scrubber solvent, thetower being coupled to first and second scrubber solvent sources whichare capable of supplying different scrubber solvents. Typically,methanol is used as a scrubber solvent in a steady state mode ofoperation, while acetic acid may be used during start-up or transientoperation of the unit. A switching system alternatively provides eithermethanol or acetic acid to the tower and returns the solvent andrecovered volatiles to the carbonylation system for further reaction.During changeover of scrubber solvents, the recovered material may beadded to the catalyst reservoir system if so desired.

The use of a one column system light ends absorber in accordance withthe invention allows operation without the need for a dedicated lightends stripper for recovering methyl iodide from the system offgas ventstream. A salient benefit is capital reduction for new acetic acidcarbonylation projects (eliminate stripper column system, reboiler,subcooler, and associated instrumentation and piping) by being able touse one absorber system on two different scrub mediums to accommodateall modes of operation including: startup, normal operation, upsetoperation and shutdown. Another benefit is energy reduction for normaloperation via steam savings realized by elimination of the need for anauxiliary stripper column for scrubber solvent. Still another benefit isbetter scrub of light ends component methyl iodide by using sub-chilled5-15° C. methanol for a normal operation mode.

Further details and advantages will be apparent from the discussionwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to theappended drawings, wherein like numerals designate similar parts. In thedrawings:

FIG. 1 is a schematic diagram illustrating a carbonylation system formaking acetic acid and

FIG. 2 is a schematic diagram illustrating a vent gas absorber andswitching system used in connection with the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in detail below with reference to numerousembodiments for purposes of exemplification and illustration only.Modifications to particular embodiments within the spirit and scope ofthe present invention, set forth in the appended claims, will be readilyapparent to those of skill in the art.

Unless more specifically defined below, terminology as used herein isgiven its ordinary meaning. % and like terms refer to weight percent,unless otherwise indicated.

“Consisting essentially of” and like terminology refers to a compositionconsisting of 90% by weight or more of the specified component. Thus ascrubber solvent stream consists essentially of methanol if it is atleast 90% methanol.

“Low pressure” and like terminology refers to pressures much lower thanthe pressure maintained in a carbonylation reactor of the classdiscussed herein. Low pressure thus refers to gauge pressures generallyless than 10 bar, suitably less than 5 bar, typically less than about 3bar and in some cases less than 1 bar.

“Volatile” components are those compounds in vapor phase and/or having aboiling point below or equal to that of methyl acetate including methyliodide.

As used herein the “purified process stream” includes the process streamfed forward from the light ends column, and any subsequent purificationsof the light ends process stream.

A Group VIII catalyst metal used in connection with the presentinvention may be a rhodium and/or iridium catalyst. The rhodium metalcatalyst may be added in any suitable form such that rhodium is in thecatalyst solution as an equilibrium mixture including [Rh(CO)₂I₂]⁻ anionas is well known in the art. When rhodium solution is in the carbonmonoxide-rich environment of the reactor, solubility of the rhodium isgenerally maintained because rhodium/carbonyl iodide anionic species aregenerally soluble in water and acetic acid. However, when transferred tocarbon monoxide depleted environments as typically exist in the flasher,light ends column and so forth, the equilibrium rhodium/catalystcomposition changes since less carbon monoxide is available. Rhodiumprecipitates as RhI₃, for example; details as to the form of entrainedrhodium downstream of the reactor is not well understood. Iodide saltshelp alleviate precipitation in the flasher under so-called “low water”conditions as will be appreciated by one of skill in the art.

Iodide salts maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal or a quaternary ammonium or phosphonium salt. Incertain embodiments, the catalyst co-promoter is lithium iodide, lithiumacetate, or mixtures thereof. The iodide salt may be added as a mixtureof salts such as a mixture of lithium iodide and sodium iodide and/orpotassium iodide. Alternatively, the iodide salt may be generatedin-situ since under the operating conditions of the reaction system, awide range of non-iodide salt precursors such as alkali metal acetateswill react with methyl iodide to generate the corresponding co-promoteriodide salt stabilizer. Suitable salts can be generated in situ fromnon-ionic precursors, such as a phosphine oxide or any suitable organicligand or ligands if so desired. Phosphine oxides and suitable organicligands generally undergo quaternization in the presence of methyliodide at elevated temperatures to yield salts which maintain iodideanion concentration. For additional detail regarding iodide saltgeneration, see U.S. Pat. No. 5,001,259 to Smith et al.; U.S. Pat. No.5,026,908 to Smith et al.; and U.S. Pat. No. 5,144,068, also to Smith etal., the disclosures of which are hereby incorporated by reference.

An iridium catalyst in the liquid carbonylation reaction composition maycomprise any iridium-containing compound which is soluble in the liquidreaction composition. The iridium catalyst may be added to the liquidreaction composition for the carbonylation reaction in any suitable formwhich dissolves in the liquid reaction composition or is convertible toa soluble form. Examples of suitable iridium-containing compounds whichmay be added to the liquid reaction composition include: IrCl₃, IrI₃,IrBr₃, [Ir(CO)₂I]₂, [Ir(CO)₂Cl]₂, [Ir(CO)₂Br]₂, [Ir(CO)₂I₂]⁻H⁺,[Ir(CO)₂Br₂]⁻H⁺, [Ir(CO)₂I₄]⁻H⁺, [Ir(CH₃)I₃(CO₂]⁻H⁺, Ir₄(CO)₁₂,IrCl₃.3H₂O, IrBr₃.3H₂O, Ir₄(CO)₁₂, iridium metal, Ir₂O₃, Ir(acac)(CO)₂,Ir(acac)₃, iridium acetate, [Ir₃O(OAc)₆(H₂O)₃][OAc], andhexachloroiridic acid [H₂IrCl₆]. Chloride-free complexes of iridium suchas acetates, oxalates and acetoacetates are usually employed as startingmaterials. The iridium catalyst concentration in the liquid reactioncomposition may be in the range of 100 to 6000 ppm. The carbonylation ofmethanol utilizing iridium catalyst is well known and is generallydescribed in the following U.S. Pat. Nos. 5,942,460; 5,932,764;5,883,295; 5,877,348; 5,877,347 and 5,696,284, the disclosures of whichare hereby incorporated by reference into this application as if setforth in their entirety.

A supported Group VIII catalyst may employed if so desired. Onepreferred system includes an insoluble polymer having pendentpyrrolidone groups which support a rhodium species. One suitablecatalyst is a poly-vinylpyrrolidone which has been crosslinked andrhodium loaded. Cross-linking can be achieved using a caustic catalystas disclosed in U.S. Pat. No. 2,938,017 or by using a cross-linkingagent such as disclosed in German 2,059,484. These references are hereinincorporated by reference. This catalyst is prepared by reacting thepolymer support with an alkyl halide and a rhodium compound. Bothreactions are readily accomplished by standard procedures and usingknown components for such reactions. For example, it is preferred tosimply add an amount of the insoluble polymer such as in powder or resinbead form to what otherwise constitutes as the homogeneous medium forthe methanol carbonylation reaction. Such carbonylation reaction mediumincludes methanol and/or methyl acetate, acetic acid and a small amountof water in a pressure vessel along with a rhodium compound and aniodide promoter as described herein. Further details appear in U.S. Pat.No. 5,466,874, the disclosure of which is incorporated herein byreference in its entirety.

Another system includes an insoluble, pyridine ring-containing polymer,and a Group VIII metal supported thereon and is known per se. The term“pyridine ring-containing polymer” used herein is intended to refer to apolymer containing substituted or non-substituted pyridine rings orsubstituted or non-substituted, pyridine-containing polycondensed ringssuch as quinoline rings. The substituents include those inert to themethanol carbonylation such as an alkyl group and alkoxy group. Typicalexamples of the insoluble, pyridine ring-containing polymers includethose obtained by reaction of vinylpyridine with a divinyl monomer or byreaction of vinylpyridine with a divinyl monomer-containing vinylmonomer, such as copolymers of 4-vinylpyridine and divinylbenzene,copolymers of 2-vinylpyridine and di-vinylbenzene, copolymers ofstyrene, vinylbenzene and divinylbenzene, copolymers ofvinylmethylpyridine and divinylbenzene and copolymers of vinylpyridine,methyl acrylate and ethyl diacrylate. Further details appear in U.S.Pat. No. 5,334,755, the disclosure of which is incorporated herein byreference in its entirety.

Methyl iodide is used as the promoter. Preferably, the concentration ofmethyl in the liquid reaction composition is in the range 1 to 50% byweight, preferably 2 to 30% by weight.

The promoter may be combined with a salt stabilizer/co-promotercompound, which may include salts of a metal of Group IA or Group IIA,or a quaternary ammonium or phosphonium salt. Particularly preferred areiodide or acetate salts, e.g., lithium iodide or lithium acetate.

Other promoters and co-promoters may be used as part of the catalyticsystem of the present invention as described in European PatentPublication EP 0 849 248, the disclosure of which is hereby incorporatedby reference. Suitable promoters are selected from ruthenium, osmium,tungsten, rhenium, zinc, cadmium, indium, gallium, mercury, nickel,platinum, vanadium, titanium, copper, aluminum, tin, antimony, and aremore preferably selected from ruthenium and osmium. Specificco-promoters are described in U.S. Pat. No. 6,627,770, the entirety ofwhich is incorporated herein by reference.

A promoter may be present in an effective amount up to the limit of itssolubility in the liquid reaction composition and/or any liquid processstreams recycled to the carbonylation reactor from the acetic acidrecovery stage. When used, the promoter is suitably present in theliquid reaction composition at a molar ratio of promoter to metalcatalyst of [0.5 to 15]:1, preferably [2 to 10]:1, more preferably [2 to7.5]:1. A suitable promoter concentration is 400 to 5000 ppm.

The carbonylation apparatus or process that is the subject of theinvention typically includes a reactive section, purification section, acatalyst reservoir system and a light ends recovery system. The presentinvention may be appreciated in connection with, for example, thecarbonylation of methanol with carbon monoxide in a homogeneouscatalytic reaction system comprising a reaction solvent (typicallyacetic acid), methanol and/or its reactive derivatives, a solublerhodium catalyst, at least a finite concentration of water. Thecarbonylation reaction proceeds as methanol and carbon monoxide arecontinuously fed to the reactor. The carbon monoxide reactant may beessentially pure or may contain inert impurities such as carbon dioxide,methane, nitrogen, noble gases, water and C₁ to C₄ paraffinichydrocarbons. The presence of hydrogen in the carbon monoxide andgenerated in situ by the water gas shift reaction is preferably keptlow, for example, less than 1 Bar partial pressure, as its presence mayresult in the formation of hydrogenation products. The partial pressureof carbon monoxide in the reaction is suitably in the range 1 to 70 bar,preferably 1 to 35 bar, and most preferably 1 to 15 bar.

The pressure of the carbonylation reaction is suitably in the range 10to 200 Bar, preferably 10 to 100 bar, most preferably 15 to 50 Bar. Thetemperature of the carbonylation reaction is suitably in the range 100to 300° C., preferably in the range 150 to 220° C. Acetic acid istypically manufactured in a liquid phase reaction at a temperature offrom about 150-200° C. and a total pressure of from about 20 to about 50bar.

Acetic acid is typically included in the reaction mixture as the solventfor the reaction.

Suitable reactive derivatives of methanol include methyl acetate,dimethyl ether, methyl formate and methyl iodide. A mixture of methanoland reactive derivatives thereof may be used as reactants in the processof the present invention. Preferably, methanol and/or methyl acetate areused as reactants. At least some of the methanol and/or reactivederivative thereof will be converted to, and hence present as, methylacetate in the liquid reaction composition by reaction with acetic acidproduct or solvent. The concentration in the liquid reaction compositionof methyl acetate is suitably in the range 0.5 to 70% by weight,preferably 0.5 to 50% by weight, more preferably 1 to 35% by weight andmost preferably 1-20% by weight.

Water may be formed in situ in the liquid reaction composition, forexample, by the esterification reaction between methanol reactant andacetic acid product. Water may be introduced to the carbonylationreactor together with or separately from other components of the liquidreaction composition. Water may be separated from other components ofreaction composition withdrawn from the reactor and may be recycled incontrolled amounts to maintain the required concentration of water inthe liquid reaction composition. Preferably, the concentration of watermaintained in the liquid reaction composition is in the range 0.1 to 16%by weight, more preferably 1 to 14% by weight, most preferably 1 to 10%by weight.

The reaction liquid is typically drawn from the reactor and flashed in aone step or multi-step process using a converter as well as a flashvessel as hereinafter described. The crude vapor process stream from theflasher is sent to a purification system which generally includes atleast a light ends column and a dehydration column. As noted earlier,the form of any catalyst metal which is entrained to the light endscolumn and beyond is not well understood; however, the entrainedcatalyst metal is lost in conventional systems.

The present invention is further appreciated by reference to FIG. 1which is a schematic diagram illustrating a typical carbonylationprocess and apparatus. In FIG. 1 there is shown a carbonylation system10 including a reactor 12 provided with a feed system 14 including amethanol surge tank 16 and a carbon monoxide feed line. A catalystreservoir system includes a methyl iodide storage vessel 20 as well as acatalyst storage tank 22. Reactor 12 is provided with a vent 24 and anoptional vent 24 a. Reactor 12 is coupled to a flash vessel 26 by way ofa conduit 28 and optionally by way of vent 24 a. The flasher, in turn,is coupled to a purification section 30 which includes a light ends orstripper column 32, a dehydration column 34 and a strong acid,silver-exchanged cation ion-exchange resin bed 36 which removes iodidesfrom the product. Instead of a silver-exchanged, strong acid cationion-exchange resin, it has been reported that anion ion-exchange resincan be used to remove iodides. See British Patent No. G 2112394A, aswell as U.S. Pat. No. 5,416,237, Col. 7, lines 54+, which teaches theuse of 4-vinylpyridine resins for iodide removal.

A gaseous purge stream is typically vented from the head of the reactorto prevent buildup of gaseous by-products such as methane, carbondioxide and hydrogen and to maintain a set carbon monoxide partialpressure at a given total reactor pressure. A very significantimprovement in processing involves minimizing or eliminating the highpressure vent from reactor 12 via line 24 to a high pressure absorberand instead utilizing a vent line such as line 24 a. When operating atlow water conditions as described herein, by-products and ventrequirements are much reduced such that non-condensables can be ventedat low pressure after flashing and stripping the light ends as is seenin FIGS. 1 and 2 while maintaining a predetermined carbon monoxidepartial pressure in the reactor. Thus, the use of a high pressureabsorber can be eliminated and/or minimized saving capital and operatingcosts.

Optionally (as illustrated in Chinese Patent No. ZL92108244.4), aso-called “converter” reactor can be employed which is located betweenthe reactor and flasher vessel shown in FIG. 1. Optionally, the gaseouspurge streams may be vented through the flasher base liquid or lowerpart of the light ends column to enhance rhodium stability and/or theymay be combined with other gaseous process vents (such as thepurification column overhead receiver vents) prior to scrubbing. Carbonmonoxide may be added directly to a converter vessel if so desired ormay be added slightly before (upstream) or after (downstream) if sodesired in order to stabilize the catalyst solution and consume anyunreacted methanol. Details of such arrangements are seen in EuropeanPatent No. EP 0 759 419 as well as U.S. Pat. No. 5,770,768 to Denis etal., the disclosures of which are hereby incorporated by reference.

These variations are well within the scope of the present invention aswill be appreciated from the appended claims and the description whichfollows.

As will be appreciated by one of skill in the art, the differentchemical environments encountered in the purification train may requiredifferent metallurgy. For example, equipment at the outlet of the lightends column will likely require a zirconium vessel due to the corrosivenature of the process stream, while a vessel of stainless steel may besufficient for equipment placed downstream of the dehydration columnwhere conditions are much less corrosive.

Carbon monoxide and methanol are introduced continuously into reactor 12with adequate mixing at a high carbon monoxide partial pressure. Thenon-condensable bi-products are vented from the reactor to maintain anoptimum carbon monoxide partial pressure. The reactor off gas is treatedto recover reactor condensables, i.e., methyl iodide before flaring.Methanol and carbon monoxide efficiencies are generally greater thanabout 98 and 90% respectively. As will be appreciated from the Smith etal. patent noted above, major inefficiencies of the process are theconcurrent manufacture of carbon dioxide and hydrogen by way of thewater gas shift reaction.

From the reactor, a stream of the reaction mixture is continuously fedvia conduit 28 to flasher 26. Through the flasher the product aceticacid and the majority of the light ends (methyl iodide, methyl acetate,water) are separated from the reactor catalyst solution, and the crudeprocess stream 38 is forwarded with dissolved gases to the distillationor purification section 30 in single stage flash. The catalyst solutionis recycled to the reactor via conduit 40.

The purification of the acetic acid typically includes distillation in alight ends column, a dehydration column, and, optionally, a heavy endscolumn. The crude vapor process stream 38 from the flasher is fed intothe light ends column 32. Methyl iodide, methyl acetate, and a portionof the water condense overhead in the light end columns to form twophases (organic and aqueous) in a receiver 42. Both overhead liquidphases return to the reaction section via recycle line 44. Optionally, aliquid recycle stream 45 from the light ends column may also be returnedto the reactor.

The purified process stream 50 is drawn off the side of the light endscolumn 32 and is fed into dehydration column 34. Water and some aceticacid from this column separate and are recycled to the reaction systemvia recycle line 44 as shown. The purified and dried process stream 52from the dehydration column 34 feeds resin bed 36 and product is takentherefrom at 56 as shown. Carbonylation system 10 uses only two primarypurification columns and is preferably operated as described in moredetail in U.S. Pat. No. 6,657,078 to Scates et al., entitled “Low EnergyCarbonylation Process”, the disclosure of which is incorporated hereinby reference. Additional columns are generally used as desired,depending on the system.

Receiver 42 is vented via line 60 to the light ends recovery system 70shown in FIG. 2 which includes a switching system 72 which has aplurality of valves and pumps in order to selectively couple system 70to scrubber solvent sources and return the used scrub solvent to thedesired point in the carbonylation system as hereinafter described. Notealso reactor 12 may be directly vented to system 70 via line 24 ifnecessary.

Light ends recovery system 70 has an absorber tower 75 which is fed withvent gas via line 80 and with scrubber solvent via line 82. Preferablythe scrubber solvent is chilled with a chiller 84 prior to being fed totower 75 wherein the solvent flows countercurrently with respect to thevent gas, absorbing methyl iodide and additional relative componentsbefore exiting the tower via return line 84 and being returned to thecarbonylation unit. The scrubbed vent gas exits the tower at 86 and isfurther processed. For example, a second stage water scrub could be usedto remove methyl acetate, methanol, acetic acid and so forth beforeflaring if so desired. Alternatively, a second stage water scrub couldbe provided in tower 75 if so desired. Preferably, more than 90% of themethyl iodide is removed from the vent gas. The scrubber fluid isgenerally chilled to a temperature of from about 5° C. to about 25° C.prior to use in the tower, with the proviso that when acetic acid isused as the scrubber solvent, the temperature of the solvent is held at17° C. or more to prevent freezing.

Switching system 72 includes a plurality of valves such as valves 90,92, 94, 96, 98 and one or more pumps 100, 102 to raise pressure in thereturn lines 104, 106, 108, 110 if needed. Feed valves 96, 98 are usedto select the scrubber solvent which may be methanol from tank 16 oracetic acid from stream 56 depending upon the mode of operation of tower75.

In steady state operation of carbonylation system 10 valve 98 is closedand methanol is fed from tank 16 through open valve 96 via line 112 tochiller 84, wherein the methanol is cooled. From the chiller, methanolis fed to tower 75, where it flows countercurrently with vent gas andsorbs methyl iodide and other volatile components therefrom beforeexiting the column via line 84. The used solvent with sorbed methyliodide is pumped back to reactor 12 or tank 16 with pumps 100, 102 vialine 106. In this mode of operation valves 92, 94 are closed and valve90 is open.

During start up or shut down of the system it may be desirable tooperate tower 75 using acetic acid as the scrub solvent. In this mode ofoperation, valve 98 is open and valve 96 is closed. Acid may be sourcedfrom product stream 56 or a tank from (TF) if so desired. The acid flowsthrough line 112 to chiller 84 where it is chilled and fed to tower 75via line 82 and scrubs the vent gas supplied via lines 60, 80 as notedabove. The acid exits the tower 75 via line 84 and is pumped back to thecarbonylation system by way of pumps 100, 102 via lines 104, 108. Inthis mode of operation of tower 75, valves 90, 94 are closed and valve92 is open so that the used acetic acid is returned to light ends column32, the dehydration column 34, or elsewhere in the purification systemfor stripping.

During changeover from one solvent to the other, such as from methanolto acetic acid, it is generally undesirable to return the scrub fluid tothe methanol feed system or light ends column since inefficienciesresult. For such, a changeover may be accomplished in from about 5 toabout 20 minutes, during which time the used scrubber solvent is fed tocatalyst reservoir 22. In changeover mode, valves 90, 92 are closed andvalve 94 is open. Thus the system is operated generally by way of (a)feeding vent gas from the carbonylation unit to the absorber tower, thevent gas including methyl iodide and optionally additional volatilecomponents; (b) supplying a first scrubber solvent to the absorbertower, the first scrubber solvent consisting essentially of acetic acid;(c) contacting the vent gas with the first scrubber solvent therebyremoving methyl iodide and optionally additional volatile componentsfrom the gas and absorbing methyl iodide and optionally additionalvolatile components into the first scrubber solvent; (d) feeding anabsorber return stream including first scrubber solvent and absorbedmethyl iodide and optionally additional absorbed volatile components tothe light ends column, the dehydration column or elsewhere in thepurification system; (e) terminating the supply of first scrubbersolvent to the absorber tower; (f) supplying a second scrubber solventto the absorber tower, the second scrubber solvent consistingessentially of methanol; (g) contacting the vent gas with the secondscrubber solvent thereby removing methyl iodide and optionallyadditional volatile components from the gas and absorbing methyl iodideand optionally additional volatile components into the second scrubbersolvent; (h) feeding an absorber return stream including first scrubbersolvent, second scrubber solvent, absorbed methyl iodide and optionallyadditional absorbed volatile components from the absorber tower to thereactor; and (i) following the transition period, continue feeding anabsorber return stream including second scrubber solvent and absorbedmethyl iodide and optionally additional absorbed volatile components tothe reactor. Feed to the absorber tower is selected by operation ofvalves 96, 98.

While the invention has been illustrated in connection with a particularapparatus, modifications to these examples within the spirit and scopeof the invention will be readily apparent to those of skill in the art.In view of the foregoing discussion, relevant knowledge in the art andreferences discussed above in connection with the Background andDetailed Description, the disclosures of which are all incorporatedherein by reference, further description is deemed unnecessary.

1. An apparatus for producing acetic acid comprising: (a) a reactor forcarbonylating methanol or its reactive derivatives, the reactorcontaining a catalyst selected from rhodium catalysts, iridium catalystsand mixtures thereof, and a methyl iodide promoter in an acetic acidreaction mixture; (b) a feed system for providing carbon monoxide andmethanol or its reactive derivatives to the reactor; (c) a flash systemconfigured to receive a stream of the reaction mixture and separate itinto (i) at least a first liquid recycle stream, and (ii) a crudeprocess stream containing acetic acid; (d) a first distillation columncoupled to the flash system, the first distillation column being adaptedto separate low boiling components including methyl iodide from thecrude process stream, and generate a purified process stream, the firstdistillation column and optionally the reactor and flash system alsooperating to generate a vent gas stream comprising volatile organiccomponents including methyl iodide; (e) an absorber tower adapted forreceiving the vent gas stream and removing methyl iodide therefrom witha scrubber solvent, the absorber tower also being coupled to first andsecond scrubber solvent sources which are capable of supplying differentfirst and second scrubber solvents; and (f) a switching system foralternatively providing first or second scrubber solvents to theabsorber tower from either the first scrubber solvent source or thesecond scrubber solvent source.
 2. The apparatus according to claim 1,wherein the first scrubber solvent comprises methanol and the secondscrubber solvent consists essentially of acetic acid.
 3. The apparatusaccording to claim 2, wherein the first scrubber solvent consistsessentially of methanol.
 4. The apparatus according to claim 1, furthercomprising a chiller coupled to the absorber tower and first and secondscrubber solvent sources for cooling the scrubber solvents.
 5. Theapparatus according to claim 1, wherein a return stream from theabsorber tower is selectively coupled to the feed system to the reactoror the first and/or second distillation columns.
 6. The apparatusaccording to claim 1, further including a catalyst reservoir system andwherein a return stream from the absorber tower is selectively coupledto the feed system to the reactor, the first and/or second distillationcolumns, or the catalyst reservoir system.
 7. The apparatus according toclaim 1, wherein the feed system includes a methanol surge tank which isconnected to the absorber tower as the first scrubber solvent source. 8.The apparatus according to claim 1, further comprising a drying columncoupled to said first distillation column adapted for receiving thepurified product stream and removing water therefrom.
 9. The apparatusaccording to claim 1, wherein the reactor is vented to the flash system.10. A method of operating an absorber tower in a carbonylation unit formaking acetic acid of the class including a reactor and a light endscolumn comprising: (a) feeding vent gas from the carbonylation unit tothe absorber tower, the vent gas including methyl iodide and optionallyadditional volatile components; (b) supplying a first scrubber solventto the absorber tower, the first scrubber solvent consisting essentiallyof acetic acid; (c) contacting the vent gas with the first scrubbersolvent thereby removing methyl iodide and optionally additionalvolatile components from the gas and absorbing methyl iodide andoptionally additional volatile components into the first scrubbersolvent; (d) feeding an absorber return stream including first scrubbersolvent and absorbed methyl iodide and optionally additional absorbedvolatile components to the light ends column and/or dehydration column;(e) terminating the supply of first scrubber solvent to the absorbertower; (f) supplying a second scrubber solvent to the absorber tower,the second scrubber solvent comprising methanol, methyl acetate ormixtures thereof; (g) contacting the vent gas with the second scrubbersolvent thereby removing methyl iodide and optionally additionalvolatile components from the gas and absorbing methyl iodide andoptionally additional volatile components into the second scrubbersolvent; and (h) feeding an absorber return stream including secondscrubber solvent and absorbed methyl iodide and optionally additionalabsorbed volatile components to the reactor.
 11. The method according toclaim 10, wherein said second scrubber solvent consists essentially ofmethanol.
 12. The method according to claim 10, further comprising thestep of chilling the first scrubber solvent prior to supplying it to theabsorber tower.
 13. The method according to claim 10, further comprisingthe step of chilling the second scrubber solvent prior to supplying itto the absorber tower.
 14. The method according to claim 10, furthercomprising mixing the second scrubber solvent and absorbed methyl iodideand optional additional absorbed volatile components with methanol or areactive derivative thereof prior to feeding the used solvent to thereactor.
 15. The method according to claim 10, wherein the vent gasincludes methyl acetate and the first and second scrubber solvents areeffective to absorb methyl acetate from the vent gas.
 16. A method ofoperating an absorber tower in a carbonylation unit for making aceticacid of the class including a reactor and a light ends columncomprising: (a) feeding vent gas from the carbonylation unit to theabsorber tower, the vent gas including methyl iodide and optionallyadditional volatile components; (b) supplying a first scrubber solventto the absorber tower, the first scrubber solvent consisting essentiallyof acetic acid; (c) contacting the vent gas with the first scrubbersolvent thereby removing methyl iodide and optionally additionalvolatile components from the gas and absorbing methyl iodide andoptionally additional volatile components into the first scrubbersolvent; (d) feeding an absorber return stream including first scrubbersolvent and absorbed methyl iodide and optionally additional absorbedvolatile components to the light ends column and/or dehydration column;(e) terminating the supply of first scrubber solvent to the absorbertower; (f) supplying a second scrubber solvent to the absorber tower,the second scrubber solvent consisting essentially of methanol; (g)contacting the vent gas with the second scrubber solvent therebyremoving methyl iodide and optionally additional volatile componentsfrom the gas and absorbing methyl iodide and optionally additionalvolatile components into the second scrubber solvent; (h) feeding anabsorber return stream including first scrubber solvent, second scrubbersolvent, absorbed methyl iodide and optionally additional absorbedvolatile components from the absorber tower to the reactor during atransition period; and (i) following the transition period, continuefeeding an absorber return stream including second scrubber solvent andabsorbed methyl iodide and optionally additional absorbed volatilecomponents to the reactor.
 17. The method according to claim 16, whereinsaid second scrubber solvent consists of methanol.
 18. The methodaccording to claim 16, further comprising the step of chilling the firstscrubber solvent prior to supplying it to the absorber tower.
 19. Themethod according to claim 16, further comprising the step of chillingthe second scrubber solvent prior to supplying it to the absorber tower.20. The method according to claim 16, further comprising mixing thesecond scrubber solvent and absorbed methyl iodide and optionaladditional absorbed volatile components with methanol or a reactivederivative thereof prior to feeding the used solvent to the reactor. 21.An improved method of operating an apparatus for making acetic acid ofthe class including a production system having a reactor containing areaction medium and a product purification train wherein the productionsystem is vented to an absorber tower, the improvement comprising: (a)carbonylating methanol or reactive derivatives thereof in the reactor inthe presence of a Group VIII metal catalyst and a methyl iodide promoterwhile maintaining a concentration of water in the reactor of from 1-10%by weight of the reaction medium and concurrently maintaining in thereactor a predetermined partial pressure of carbon monoxide; (b) ventingnon-condensables from the production system so as to provide lowpressure vent gas only; (c) feeding the low pressure vent gas from theproduction system to the absorber tower, the vent gas including methyliodide and optionally additional volatile components; (d) supplying ascrubber solvent to the absorber tower, the scrubber solvent comprisingmethanol, methyl acetate or mixtures thereof; (e) contacting the lowpressure vent gas with the scrubber solvent thereby removing methyliodide and optionally additional volatile components from the gas andabsorbing methyl iodide and optionally additional volatile componentsinto the scrubber solvent; and (f) feeding an absorber return streamincluding scrubber solvent and absorbed methyl iodide and optionallyadditional absorbed volatile components to the reactor.
 22. The methodaccording to claim 21, wherein the Group VIII metal catalyst is asupported catalyst.
 23. The method according to claim 22, wherein theGroup VIII metal catalyst is a supported rhodium catalyst.
 24. Themethod according to claim 23, wherein the rhodium catalyst is supportedon a crosslinked polyvinylpyrrolidone polymer.
 25. The method accordingto claim 23, wherein the rhodium catalyst is supported on a crosslinkedpolyvinylpyridine polymer.